The 'suggested' perfomance of a given pipe is just a general guildline. It's not set in stone. Even within a give pipe there are single chamber, duel chamber and baffled or non baffled variations. You could have 4 versions of a .053 pipe and each will have different charictaistics. The biggest and most common mistake that folks make about pipes is thinking that a given pipe will change the performace curve of a given engine. Sadly they are right in the wrong direction.

Any given engine has it's own performance curves. Some engines are lowend and some are highend. Some are in between there somewhere. Generally a 3-4 port engine will be a lowend, high torque engine. An 8-11 port engine will generally be a highend low torque engine. "torque" always refering to the bottom end. Where the problems come in is most rookies believe that if they are suffering lowend performance with their engine then putting a "lowend" pipe on will fix it. Wrong. Putting a lowend pipe on a highend engine just chokes the highend range of the engines powerband. So now you have an engine that stinks all around. Many will argue with that remark stating, "I put a .*** on my engine and it really made it come alive!" Usually what they don't say is 'after I retuned the engine to run with the pipe I stuck on it'.

Proper way of matching a pipe/engine is to choose a pipe that makes the most of what the engine has to offer. Highend engine/ highend pipe. Lowend engine/lowend pipe. Match the pipe to the engine so that the engine makes the greatest power in it's natural power band. Once you've have done that then adjust the clutchs and gearing to take full advantage of the full power of the engine.
Don't try to MAKE the engine fit the clutch/gearing of the vehical.

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Since micro 2 stoke engines don't have any valves they use tuned pipes to simulate a valve. Each time the piston uncovers the exaust port the exaust pressure leaves the combustion chamber travels down the header into the pipe, through the pipe to the end of the pipe. Since the stinger is much smaller than the engine's exaust port then not all of the exaust pressure is relieved. This remaining exaust "pulse" then rebounds backward up through the pipe to the exaust port openning.

Timing is critical for max performance but in effect if everything is correct then that rebounding exaust pulse should reach the exaust port right about the same time as the piston is on it's way back up on it's compression stroke. When that happens then for a moment the rebound pressure is equal to the intake pressure so the exaust port simulates being closed. Now the critical part is if the engine is designed to have earlier or later compression timing then that exaust pulse will either be early or late. If it's early then the incomming fresh fuel/air charge will be short and some of the exuast gases will bounce back into the chamber. You will get a hotter less powerful power stroke. If the rebound pulse is late then some of the incomming charge will pass right through the engine and be wasted and the combustion chamber will be overly cooled by too much new fuel and no preburned fuel to help pre-heat the next charge.

Again, timing is critical to get maximum performance. Any micro engine will run with any pipe or muffler. Or even without one. But they will NOT run correctly or make power well.

So, if you have an engine that is designed to make it's max power in the topend range and you stick a lowend pipe on it then the rebounding pressure pulse is going to be late and out of time when the engine gets up into the high rpm range. Yet since the engine is designed for topend performance then a lowend pipe will not change the natural design of the engine. The engine will tend to always run rich on the topend so you lean it out. That improves the bottom end which gives the impression of improved performance. But the engine will not be making it's peak power ever.

As for the pipe shape. Yes the shape of the "convergance cone and the far end and the "divergance" cone and the near end (the header outlet is the referance point) has a marked affect on the amount of pressure that the return pulse generates. But again you really want to try to enhance the strongest points of the engine and then gear to the engine. Often if you are unlucky you may have to try 4-5 different pipes to find the one that makes the most of the engine. If you are real lucky you run into someone that has already found the optimum combo for your brand of engine and they save you a lot of trial and error. Or you pick a brand that has a matched combo already like OS or RB.